Method of manufacturing rod-shaped crystals of semi-conductor material



y 3, 1965 J. A. M. DIKHOFF 3, 4,

METHOD OF MANUFACTURING ROD-SHAPED CRYSTALS OF SEMI-CONDUCTOR MATERIAL Filed Sept. 2, 1960 5 36 FIG. 6

INVENTOR United States Patent Claims priority, application Netherlands, Sept. 18, 1959,

4 Claims. in. 148--1.6)

The invention relates to a method of manufacturing rod-shaped crystals of semi-conductor material having diamond structure by growing a pre-orientated seed crystal from a melt of the material. The term diamond structure as used herein is to be understood to mean not only a crystal structure comprising one kind of atoms, such as in germanium and silicon, but also a crystal structure comprising several kinds of atoms and exhibiting an analogous arrangement of the atoms in the crystal lattice, such as, for example, the sphalerite or zinc blende lattice. Growing may be effected by various methods, the most usual being the crystal pulling method, the zone melting method and the zone melting method without the use of a crucible, the so-called floating zone method. The rod-shaped, generally monocrystalline crystals obtained can be divided, for example by sawing, into plate-shaped bodies which are used in semiconductor electrode systems, such as transistors, diodes and photoelectric cells.

Such a rod-shapedcrystal is required to be homogeneous as far as possible, at least with respect to its specific resitivity. It has been found in practice that in such a rod-shaped crystal the specific resistivity in the transverse directions frequently is not homogeneous, and that in particular the phenomenon may occur that in a generally central part of the crystal extending in the direction of length thereof the specific resistivity differs considerably from that in the adjoining parts of the crystal. Due to this effect, which hereinafter will be referred to as core formation, the specific resistivity values measured at different points through cross sections of the crystal show relative differences exceeding 30% and even 50%. The term relative difference between two specific resistivity values is use-d herein to denote the ratio between the difference and one half of the sum of these specific resistivity values expressed as a percentage. The cause of this core formation was not known. It has already beensuggested to reduce the said effect by rotating and/ or vibrating the growing crystal; however, it has been found. that the said effect is not sufficiently counteracted thereby.

It is an object of the invention to improve the homogeneity of the crystal, in particular with respect to its specific resistivity and to prevent the occurrence of core formation.

The invention is based on the discovery that the occurrcnce of core formation is related to the orientation of the lattice of the growing crystal. Since generally in semiconductor elect-rode systems plate-shaped single-crystal semi-conductor bodies are used in which the normals to the upper and lower faces coincide with a main crystallographic, direction of the crystal lattice, that is to say a [l(l0]-, [1101- or [UH-direction, and preferably with a [UH-direction, in practice in manufacturing rodshaped crystals the direction of growing is generally chosen as accurately as possible according to such a main crystallographic direction, in most cases according to a [UH-direction, the obtained rod-shaped crystal being subsequently divided, for example by sawing, into slices according to planes extending at right angles to the longitudinal direction of the rod. Furthermore, the invention "ice is based on the discovery that core formation takes place at the point at which in the growing crystal the normal to the solidification frontier or solid-liquid interface coincides with a main crystallographic direction of the crystal, the strongest core formation occurring when this normal coincides with a [MU-direction, and that this undesirable effect can be avoided by choosing the orientation so that at no point the main crystallographic directions are at right angles the to the solidification frontier.

Instead of ensuring in the usual manner that one of the main crystallographic directions of the seed crystal coincides as accurately as possible with the direction of growing, according to the invention the seed crystal is orientated so that the main crystallographic directions of the crystal deviate from the growing direction of the crystal by angles of at least 5. Thus, the occurrence of core formation in the growing crystal can be avoided. Afterwards, the resultant rod-shaped crystal is sliced into wafers by dividing the rod along planes extending substantially at right angles to one of the main crystallographic directions of the crystal. In general, the solidification frontier of the growing crystal tends to extend in a direction such that locally the normal to this frontier coincides with a [UH-direction, this tendency increasing with increased curvature of the frontier. In order to make certain that the normal to the solidification frontier at all points deviates sufficiently from a [111]- direction, the minimum angle between the growing direction and the [UH-directions of the crystal preferably are chosen so as to exceed 5 and even to be at least 20.

Preferably the smallest of the angles between the growing direction and the [UH-directions does not exceed 30, so that the rod-shaped crystal produced can readily be divided into slices according to dividing planes extending at right angles to the [UH-direction associated with this smallest angle.

The invention will now be explained more fully with reference to a drawing, in which:

FIGURE 1 shows an apparatus for growing a crystal to a seed crystal by means of the floating-zone method,

FIGURE 2 shows the orientation of'the crystal of FIGURE 1,

FIGURE 3 shows an apparatus for pulling a crystal from a melt,

FIGURE 4 shows the orientation of the crystal of FIGURE 3,

FIGURE 5 shows an apparatus for zone melting in an elongated crucible, and

FIGURE 6 shows the orientation of the seed crystal of FIGURE 5.

In FIGURE 1, reference numeral 1 designates a silicon seed crystal the upper end of which is secured in a holder 2 revolving about its vertical axis X at a speed of revolutions per minute. A silicon rod 3 arranged vertically along the X-axis is secured at its lower end in a holder 4. Between the rod 3 and the crystal 1 aligned therewith is a molten zone 5 which'is heated by a highfrequency coil 6 symmetrically encircling this zone, themolten material being prevented from flowing down by its high surface tension. The high-frequency coil islowered in the direction indicated by an arrow at a rate of 1 mm. per minute, so that the rod 3 melts gradually and the crystal 1 gradually grows in the vertical direction so as to form a rod. The solidification frontier 7 between the growing seed crystal 1 and the molten zone 5 has a slightly curvedform which is substantially symmetrical with respect to the X-axis. The orientation of the crystal 1 is shown in FIGURE 2 by means of a cube 8 the position of which corresponds to that of a cubic unit cell of the crystal. The [lIIJ-direction 9 (a cube body diagonal) must be assumed to extend parallel to the plane of the drawing.

It makes an angle of 25 with the growing direction 10 of the crystal. The other, not shown [UH-directions (the other cube body diagonals) make larger angles with the growing direction, while the [100] -directions (extending according to the edges of the cube) and the [110]- directions (which extend according to the face diagonals of the cube but are not shown) make angles with the growing direction exceeding 5. Thus, there is no core formation in the growing crystal. It is understood that when the term HIM-directions is employed'herein, it includes any and all directions within the cubic crystal corresponding to a cube diagonal, of which there are four, totalling eight directions; and the term [100]- directions similarly indicates any and all directions corresponding to a cubic crystal edge; and the term [110]- directions similarly includesany and all directions corresponding to a face diagonal of the cubic crystal. The term main crystallographic directions means all the directions corresponding to one of the foregoing [111]-, [100]- or [110] -directions.

The rod-shaped crystal obtained in this manner can readily be divided into slices by sawing according to planes at right angles to the [UH-direction 9. If in such a slice the specific resistivity is measured at different points, it

is found that the measured values may differ from one another, but, the relative differences are always less than FIGURE 3 shows a device for pulling crystals from a melt, reference numeral 11 denoting a graphite crucible arranged on a graphite disc 12 having a rim 13 and disposed on a container 14, so that two chambers 15 and 16 are produced between the crucible 11 and the container 14. The container 14 comprises a cylinder 17 and a piston 18. By means of a narrow duct 19, which is provided in the bottom of the crucible '11 and the upper wall of the container 14 and extends through a rod-shaped intermediate member 20 passing through an aperture 2.1 in the disc 12, the container 14 is in communication with the crucible 11. The assembly is surrounded by a highfrequency coil 22 so that it can be heated. Due to the presence of the graphite disc 12 with the rim 13 and of the chambers 15 and 16, additional heat can be supplied to the bottom of the crucible 11. V

The container 14 and the duct 19 are entirely filled with a melt 23 of germanium, which also fills the crucible 11 to r is measured at different points, the measured values may be different but the relative differences are always less than 10%.

FIGURE 5 shows an apparatus for producing rodshaped crystals by means of zone melting, in which a quartz crucible 3%? in the form 'of an elongated boat is adapted to be displaced horizontally in its direction of length with respect to-an encirclinghigh-frequency coil 31. The crucible'contains a germanium rod 32 and a germanium seed crystal 33 having a 1l1]-direction 36 extending along its longitudinal axis. The seed crystal 33 leans against an end wall 34 of the crucible so'that its direction of length makes an angle of 10 with the direction of length of the crucible 30. The orientationrof the seed crystal is shown in FIGURE 6 with the aid of a cube 35,'while the [l11]-direction 36 must be assumed to extend parallel to the plane of the drawing.

The crucible (FIGURE 5) now is arranged so that the high-frequency coil 31 surrounds the area of contact between the rod 32 and the seed crystal 33 'and subse quently a molten zone 37 is produced at this point byv the coil being energized; Then the crucible is displaced with respect to the coil in the direction indicated-by an arrow at a rate of 2 mm. per minute, so that the molten zone passes through the rod 32 in the direction of length thereof, in which process this rod gradually. melts and the seed crystal 33 grows along the direction of length of the cru cible to form a rod-shaped crystal-having an orientation as shown in FIGURE 6. Since at the upper; surface of the zone 37 (FIGURE 5 heating is more intense than at the lower surface, the; upper surface of the zone is wider than its lower surface,-so that the solidification frontier'38 is asymmetrical with respect to the longitudia height of only 6 mm. measured from the bottom of a the crucible. A rod-shaped growing germanium crystal 24, 2 cms. in diameter produced by partial growth of a pro-oriented seed crystal, is drawn vertically from the melt in the crucible at a uniform speed of 1 mm. per minute while rotating about its Y-axis at a speed of revolutions per minute and while gradually growing further. By gradually raising the piston 18 the level of the melt in the crucible is maintained constant.

Owing to the additional heating of the bottom of the crucible and the slight spacing between the lower face of the crystal 24 and the bottom, at all points of the solidification' frontier 25 of the growing crystal there is a substantially vertical heat transport, whereas the heat transplane of the drawing and makes an angle of 15 with.

the growth direction 28 of the crystal, while the other main crystallographic directions make larger angles therewith. The.cr.ystal produced can be divided into slices according to planes extending at right angles to the [111]- direction 27. If, now, in such a slice the specific resistivity nal axis of the growing part of the crystal, the lower part of this frontier being substantially at right angles to a direction 39 in which the crystal grows whereas the upper portion of the frontier greatly deviates from this position. Since the [11l]-direction 36 of the seed crystal deviates from the growth direction 3? (FIGURE 6) through an angle of 10, at no point is it at right angles to the solidification frontier 38 (FIGURE 5) sothat no core formation occurs in the growing crystal. It should be mentioned that in this case the choice of the direction in which the [111]-axis'36 deviates from the growing direction by a comparatively small angle is not arbitrary but directly related to the asymmetrical shape of the solidifi: cation frontier.

What isclaimed is 1. A method of making generally monocrystalline wafers for use 'in semiconductor devices, comprising providing a single seed crystal of semiconductormaterial having a diamond structure and having three main crystiallograpihic directions including the [flu-directions, []-directions, and the []-directions, contacting the seed to a melt of the same semiconductor material which is then translated away. relative to the seed to grow from the seed agenerally rod-shaped crystal, said single seed crystal being oriented relative to the-melt such that its growing direction corresponding to the direction of translation deviates from-all of the said three main crystallographic directions-by an angle of at'least 5, and the smallest angle between a [UH-direction of the seed and the said growing direction. does not exceed 30, and such that the melt-solid interface is oblique to the said three main crystallographic directions and the rodshaped crystal thus grown exhibits more uniform transverse resistivity, and forming monocrystalline wafers from the thusgrown rod-shaped crystal by dividing same along planes extending substantially at rig-ht angles to one of said three main crystallographic directions, said wafers or sections thereof to be incorporated in the devices made.

2. A method as set forth in claim 1 wherein the crystal is grown by pulling the oriented seed from a melt.

3. A method as set forth in claim 1 wherein the crys tal is grown by zone-melting from the oriented seed.

4. A method of making generally monocrystalline wafers for use in semiconductor devices, comprising providing a single seed crystal of semiconductor material having a diamond structure and having three main crystallographic directions including the [111]-directions, [1001-directions, and the [110]-directions, contacting the seed to a melt of the same semiconductor material which is then translated away relative to the seed to grow from the seed a generally rod-shaped crystal, said single seed crystal being oriented relative to the melt such that its 10 growing direction corresponding to the direction of translation deviates from all of the said three main crystallographic directions by an angle of at least 5, and the smallest angle between a [UH-direction of the seed and the said growing direction is between 20 and 30, and such that the melt-solid interface is oblique to the said three main crystallographic directions and the rod-shaped crystal thus grown exhibits more uniform transverse resistivity, and forming monocrystalline wafers from the thus-grown rod-shaped crystal by dividing same along planes extending substantially at right angles to the said 6 [UH-direction forming the said smallest angle, said wafers or sections thereof to be incorporated in the devices made.

References Cited by the Examiner UNITED STATES PATENTS 2,651,831 9/53 Bond et al 2925.3 2,981,875 4/61 Kelley et al 317-235 3,018,539 1/62 Taylor et a1 2925.3

OTHER REFERENCES Growth Twins in Germanium, by Bolling et a1., Canadian Journal of Physics, vol. 34, Jan-June 1956, pages 234-240.

Karstensen: Journal of Electronics and Control, vol. 3, July-Dec. 1957, pages 305-307.

NORMAN YUDKOFF, Primary Examiner.

Examiners. 

1. A METHOD OF MAKING GENERALLY MONOCRYSTALLINE WAFERS FOR USE IN SEMICONDUCTOR DEVICES, COMPRISING PROVIDING A SINGLE SEED CRYSTAL OF SEMICONDUCTOR MATERIAL HAVING A DIAMOND STRUCTURE AND HAVING THREE MAIN CRYSTALLOGRAPHICH DIRECTIONS INCLUDING THE (111)-DIRECTIONS, (100)-DIRECTIONS, AND TEH (110)-DIRECTIONS, CONTACTING THE SEED TO A MELT OF THE SAME SEMICONDUCTOR MATERIAL WHICH IS THEN TRANSLATED AWAY RELATIVE TO THE SEED TO GROW FROM THE SEED A GENERALLY ROD-SHAPED CRYSTAL, SAID SINGLE SEED CRYSTAL BEING ORIENTED RELATIVE TO THE MELT SUCH THAT ITS GROWING DIRECTION CORRESPONDING TO THE DIRECTION OF TRANSLATION DEVIATES FROM ALL OF THE SAID THREE MAIN CRYSTALLOGRAPHIC DIRECTIONS BY AN ANGLE OF AT LEAST 5*, AND THE SMALLEST ANGLE BETWEEN A (111)-DIRECTION OF THE SEED AND THE SAID GROWING DIRECTION DOES NOT EXCEED 30*, AND SUCH THAT THE MELT-SOLID INTERFACE IS OBLIQUE TO THE SAID THREE MAIN CRYSTALLOGRAPHIC DIRECTIONS AND TEH ROD-SHAPED CRYSTAL THUS GROWN EXHIBITS MORE UNIFORM TRANSVERSE RESISTIVITY, AND FORMING MONOCRYSTALLINE WAFERS FROM THE THUSGROWN ROD-SHAPED CRYSTAL BY DIVIDING SAME ALONG PLANES EXTENDING SUBSTANTIALLY AT RIGHT ANGLES TO ONE OF SID THREE MAIN CRYSTALLOGRAPHIC DIRECTIONS, SAID WAFERS OF SECTIONS THEREOF THREE INCORPORATED IN THE DEVICES MADE. 